1. Field of the Invention
The invention relates to a contact pressure mechanism for pressing or lifting a web-shaped image receiving material against or off of a drum-shaped image carrier in a toner image transfer station of an electro-photographic recording or copying device.
2. Description of the Prior Art
In fast printers such as laser printers which operate on the principle of electrophotography, continuous forms are printed with a continuous speed. At the transfer location, a powder image situated on a photo-conductor is thereby transfered onto the paper. For the purpose of printing, the paper must be swiveled against the photo-conductive drum and must contact the latter in order to accept the information. The contact between paper and photo-conductor must thereby be interruptable and restorable. To this end, the required paper feed means must meet the following function demands:
The approach of the paper to the photo-conductor as well as the interruption of the contact between photo-conductor and paper ensues while the photo-conductor and the paper are moved with identical and uniform speed. These operations must be executed very fast because no powder image can be transferred during these phases, and, thus, a non-printable region which should be kept as small as possible arises on the paper.
Due to the required rapidity of motion, mass forces arise in the paper and these can lead to widening of the transport perforations given the employment of standard margin-punched continuous paper. The demand therefore exists that only a small portion of the paper web dare be moved toward and away from the photo-conductor. Particularly when the paper web reapproaches the photo-conductor after an interruption in printing, parts of the paper web coated with toner which are already provided with a powder image again come into contact with the moving photo-conductor. No or only an extremely slight relative motion between paper and photo-conductor dare therefore occur during the in-swiveling. This would otherwise lead to a smearing of the powder image. Such a relative motion also dare not appear when the paper web is stripped from the photo-conductor if paper coated with toner is still situated in the region of the photo-conductor.
German Pat. No. AS 29 03 265 discloses an apparatus for the transport of an image receiving material through an image transfer station and for in/out-pivoting of the image receiving material against/from an intermediate image carrier in the image transfer station of a copying device and non-mechanical recording device.
The movement of the paper web toward the photo-conductor is produced by a rotatably seated swivel strap which is moved by a lifter magnet. Following the transfer station, the paper is placed into a pair of feed beads by means of transport perforations and is tensed by means of a spring brake attached preceding the transfer station.
When the paper web is swiveled away from the photo-conductor, the length of paper becoming free is transported away via the pair of feed beads and the required length is pulled out via the spring brakes during in-swivel. The paper web thus always remains tensed and a smearing of the powder image is not possible. The high mechanical stressing of the paper web and, thus, of the transport perforations does, however, represent a problem.
German Pat. No. 31 27 777 further discloses a means for tensing a recording medium moved by means of feed tractors wherein the image receiving material is situated in positive engagement with conveyor beads arranged preceding and following the transfer station.
German Pat. No. 31 28 983 describes a contact pressure mechanism for a web-shaped receiving material.
The contact pressure mechanism actuated by a lifter magnet for pressing or lifting a web-shaped image receiving material against or off from an intermediate image carrier employs a contact pressure rail extending over the width of the image receiving material perpendicular to its feed direction, this contact pressure rail being pivotable around a laterally arranged shaft.
In a first embodiment, a tappet is rigidly connected to the contact pressure rail, this tappet being rotatably and displaceably seated in a slot of a dog which proceeds perpendicular to the moving direction of the lifter magnet pin. In a second embodiment, the linear lifting motion of the dog is first converted into a rotary motion of a cam plate which in turn effects the swivel motion of the contact pressure rail.